According to the current recommendations, the best time to sow winter rape in Poland is between August 5 and 25. Should sowing outside this period be considered a risky agrotechnical mistake or rather a new norm? Should practice follow theory, or maybe vice versa?
(Not) current recommendations for rapeseed sowing
The sowing date of winter rapeseed determines the growth and development of plants in late summer and autumn, and thus the condition with which they enter winter dormancy and resume vegetation in spring. The correct sowing date therefore largely determines the seed yield that we will obtain from the plantation. An important criterion for determining the optimal sowing date is the thermal requirements of winter rapeseed. It is estimated that from sowing to the cessation of vegetation in autumn, rapeseed should "accumulate" a sum of effective temperatures at the level of 800-850°C. With this knowledge and based on the knowledge of approximate average daily temperatures, it was determined that plants need 75-80 days of full vegetation with temperatures above 5°C before cessation of vegetation for the winter. Depending on the region of the country and the prevailing climatic conditions, the optimal sum of temperatures is ensured by sowing between 5 and 25 August.
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However, here the first "but" is born. The currently known recommendations for the optimal sowing date of winter rape in Poland were formulated several dozen years ago, and during this time climatic conditions have changed significantly. It is enough to refer to the data of the Institute of Meteorology and Water Management: from the beginning of the measurement archive in the 1950s to the end of the 1980s, the average air temperature values in individual decades were in the range of 7.5-8°C. In the decade 2011-2020 it was already an average of 9.33°C. It is estimated that the increase in the average air temperature in Poland is 0.29°C per decade. These changes are not without impact on the vegetation period and plant development. We observe this more and more often on plantations, for example in the form of mild winters and creeping vegetation. Several dozen years ago, winter plant dormancy lasted on average from mid-November to mid-March – today this is no longer the rule.
Sowing rapeseed at the theoretically optimal time does not always bring the best results
In recent years, observations by rapeseed producers have increasingly indicated that sowing at the theoretically optimal date does not always bring the best results. With increasingly warmer autumns and winters, rapeseed sown according to recommendations may accumulate too high a sum of effective temperatures, which will translate into rosette luxuriance, worse winter hardiness and the need to incur greater costs for plant regulation. In the last few seasons, farmers and advisors have been warning about the increasing autumn pressure of agrophages on plantations established at the time, which generates further costs for protection, the possibilities of which are, by the way, limited. More and more farmers start sowing rapeseed in September. Sometimes it is a conscious choice, and sometimes it is forced, e.g. due to the accumulation of field work or the extended waiting period for a service provider or due to unfavourable moisture conditions in the soil. Rapeseed sown in September turns out to be easier to manage in many cases, winters satisfactorily and yields at a similar level to that sown at the optimal date. We are increasingly hearing voices saying that the current recommendations are no longer up to date and require verification and re-establishment.
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Capricious weather and rapeseed sowing
On the other hand, the course of weather conditions in Poland can be capricious, and one season can call into question previous conclusions and newly acquired habits. The climate does not change linearly, systematically year after year. There are still years in which the cooling comes exceptionally early or lasts for a very long time. Considering the high variability of weather conditions in individual years, it may turn out that in a given season a new, later sowing date will be detrimental to the farmer. In the case of too late sowing in a given year, rapeseed will not develop the appropriate number of leaves in the rosette and the root collar with the optimal diameter before winter, and the root will not grow through the soil well. This may consequently worsen wintering and lead to a decrease in yield of up to 50-60 kg of seeds for each day of delayed sowing.
Proposals for new rapeseed sowing dates
In terms of the established optimal sowing dates for winter rape, the area of Poland has been divided into four cultivation regions, which roughly cover: the strict north-east of the country (mainly the Podlaskie Voivodeship and the eastern part of the Warmian-Masurian Voivodeship), the eastern belt, the central belt and the western and south-western parts of the country. Traditionally, the earliest sowing – from 5 August – is recommended in the east of the country, gradually delaying it towards the west until 25 August. However, rapeseed producers and an increasing number of advisors and breeding and seed companies are slowly allowing the sowing date to be postponed by an average of 5-7 days in relation to the recommendations. In practice, there is also no shortage of September sowing, but for safety reasons, only hybrid varieties are recommended for these dates, which, thanks to their good initial vigour, are able to quickly “catch up” in development. In the case of delayed sowing, it is also possible to decide to slightly increase the sowing rate, although it is worth adjusting the decision individually to the recommendations of the breeder of a given variety.
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Drought drains state budget and farmers' pockets
The Central Research Centre for Cultivated Plant Varieties decided to address the issue of the need to update the recommended sowing dates for winter crops in Poland during a joint, off-site meeting of the Commission for the Registration of Oil and Fiber Plant Varieties and the Commission for the Registration of Cereal Plant Varieties, which took place on May 24, 2024 in Smolice. The premises for changing the recommendations and proposals for new agrotechnical dates were presented by Andrzej Najewski, head of the Workshop on the Economic Value of Cereal Plant Varieties, and Jacek Broniarz, head of the Workshop on Fodder, Oil and Fiber Plants at COBORU. In the case of winter rape, the recommended sowing dates would be postponed or extended by 5-10 days compared to the recommendations in force so far. It should be noted that these are only proposals for now. Scientists associated in COBORU Commissions pointed out that changes in recommendations should be approached with great caution and noticed the need to differentiate recommendations for population and hybrid varieties. It seems, therefore, that we should wait a bit longer for the official introduction of new optima and adjust them so that they are completely safe.
